The present invention relates to magnetic iron oxide particles, a process for producing such particles and a magnetic recording medium. More particularly, the present invention relates to magnetic iron oxide particles showing black in color, having a high coercive force and being also excellent in magnetic and chemical stability, a process for producing such particles, and a magnetic recording medium comprising a substrate and a magnetic recording layer containing the said magnetic iron oxide particles.
With a recent tendency toward miniaturization and lightweight of the magnetic recording and reproduction device, requirements are increasing for higher performance of the magnetic recording media such as magnetic tapes and discs. Such magnetic recording media are generally produced by coating a resin composition containing magnetic particles on a substrate, and it is known that the properties of the magnetic recording media are closely concerned with the magnetic particles used. It is required of the magnetic recording media to be not only high in recording density, sensitivity and output, but also excellent in magnetic and chemical stability. For satisfying these requirements on magnetic recording media, it is necessary that the magnetic iron oxide particles used for the said recording media possess a high coercive force, are colored in black and also have an excellent magnetic and chemical stability.
This fact is described in Japanese Patent Publication (Kokoku) No. 55-6580 (1980) as "In recent years, there has been a tendency that the wavelength of the recording signals shifts into the short wavelength region, and this tendency is remarkable with the video cassette tape recording media. Thus, the magnetic recording media are required to be enhanced in recording density and output, especially improved in frequency characteristics and magnetic stability. For satisfying these requirements on magnetic recording media, the magnetic material used therefor demands to have magnetic stability and high coercive force (Hc)."
Also, for improving recording density characteristics of magnetic recording media, it is necessary to enhance coercive force of magnetic recording media, and for that reason, the magnetic particles are required to have as high a coercive force as possible. This fact is described in "Electronic Communication Society Technical Research Reports" MR77-36 (1978) page 37 as "In order to increase recording density of magnetic tapes, it is necessary to enhance coercive force of the magnetic particles used in the tapes."
Co-doped type magnetic iron oxide particles and Co-coated type magnetic iron oxide particles are known as the magnetic particles having high coercive force. The Co-doped type magnetic iron oxide particles can be obtained by adding a Co salt in the course of the reaction for producing magnetite particles, or by adding a Co salt in the course of the reaction for producing goethite particles and then reducing the obtained Co-containing goethite particles to produce the Co-containing magnetite particles, and if necessary, further oxidizing to produce the Co-containing magnetite particles. The Co-coated type magnetic iron oxide particles can be obtained by coating the surface of precursor particles comprising magnetite particles obtained by reducing the goethite particles which are the starting material or comprising magnetite particles obtained by oxidizing the said magnetite particles, with a Co compound.
As is well known in the art, the Co-doped type magnetic iron oxide particles are very unstable magnetically and chemically.
Co-coated type magnetic iron oxide particles, as compared with Co-doped type particles, are fairly excellent in magnetic and chemical stability, and especially those obtained by using magnetite particles as precursor particles, as compared with those obtained by using magnetite particles as precursor particles, have a high coercive force and a high saturation magnetization, and are relatively stable against pressure, so that the Co-coated type magnetite particles are expected as magnetic iron oxide particles for magnetic recording.
It is known, however, that Co-coated type magnetite particles, although stable as compared with Co-doped type magnetic iron oxide particles, are still generally unstable magnetically and chemically due to the presence of Fe.sup.2+ in the particles. This phenomenon is described in Japanese Patent Publication (Kokoku) No. 55-6580 (1980) as "Cobalt-containing acicular magnetite particles are high in both coercive force and saturated flux density . . . and their use as magnetic material for magnetic recording is expected, but on the other hand, they have the defect that a change of coercive force with the passage of time is large as they contain Fe.sup.2+." and ". . . when the cobalt-containing acicular magnetite particles are taken out into the air, they are oxidized to reduce Fe.sup.2+ therein less than the stoichiometrical amount, causing formation of defects in the crystal lattice. When these cobalt-containing acicular magnetite particles are allowed to stand at room temperature, there takes place migration of the cat ions (Fe.sup.2+, Co.sup.2+) to the stable positions through the defects, causing gradual increase of coercive force with the passage of time . . . . The change of coercive force with the passage of time, when the cobalt-containing acicular magnetite particles are left at room temperature, is associated with the coordination of ions in the crystal lattice . . . . When they are allowed to stand at room temperature, there takes place migration of Fe.sup.2+ and Co.sup.2+ to the stable positions through the defects, and as a result there occurs the change of coercive force with the passage of time."
Thus, when the Co-coated magnetite particles are heated to a certain temperature, the coercive force of the particles changes after heating and the coercive force after cooling to room temperature is not the same as that before heating. When the Co-coated magnetite particles are allowed to stand for a certain period of time, there take place such phenomena as the increase of coercive force with the passage of time, the reduction of saturation magnetization with the passage of time (hereinafter referred to as magnetic unstability) and the decrease of Fe.sup.2+ with the passage of time (hereinafter referred to as chemical unstability).
Thus, in the case of the magnetic recording media obtained by using Co-coated type magnetite particles, when heated to a certain temperature, the coercive force after heating is not the same as that before heating, and when allowed to stand for a certain period of time, there take place such phenomena as the increase of coercive force and squareness (Br/Bm) with the passage of time, the decrease of saturated flux density Bm with the passage of time, and the increase of light transmittance with the passage of time.
On the other hand, stoppage of running of magnetic recording media such as magnetic tapes, especially video tapes, is effected by detecting a high light transmittance portion of the recording medium by a video deck. Recently, with enhancement of performance of the magnetic recording media, there has been an increasing technological tendency toward smaller thickness of magnetic recording media and smaller particle size of magnetic iron oxide particles dispersed in the recording layer, and as a result, there arises the problem that light transmittance of the magnetic recording layer is increased and it is difficult to detect the high light transmittance portion by the video deck.
As a solution to this problem, it is tried to add carbon black powder in the magnetic recording layer, thereby reducing light transmittance. Thus, in manufacture of the current video tapes, it is essential to add carbon black powder in the magnetic recording layer. In manufacture of magnetic recording media, however in the case where carbon black powder is added when forming a coating composition by mixing the magnetic particles and a binder resin, there arises the problem that dispersion of the magnetic particles in the vehicle is disturbed, causing deterioration of orientation and packing property of the magnetic particles in the coat. Further, carbon black powder has difficulties in handling because of low bulk density, which is around 0.1 g/cm.sup.3 and thus is bad in workability Also, the problems of safety and sanitation, such as carcinogenicity, are pointed out. When using the magnetic particles containing a superparamagnetic iron oxide disclosed in Japanese Patent Publication (Kokoku) Nos. 58-18766 (1983) and 60-12286 (1985), the dispersibility in the vehicle is improved but the effect of reducing the light transmittance is unsatisfactory.
Thus, addition of carbon black powder which does not participate in magnetism is contrary to the purpose of elevating performance of magnetic recording media, so that it is tried to reduce the carbon powder content as much as possible by using Co-coated magnetite particles which have a higher blackness degree than Co-coated magnetite particles, as the magnetic iron oxide particles to be dispersed in the magnetic recording layer.
It is known that the blackness degree of Co-coated magnetite particles is influenced mainly by Fe.sup.2+ content and the blackness degree tends to rise as Fe.sup.2+ content increases. This fact described in "Powders and Powder Metallurgy", Vol. 26, No. 7, pp. 239-240, as "The blackness degree of the sample is affected by Fe(II) content and average particle size, and the particles with average particle size of 0.2 .mu.m is bluish black and is most suited for use as black pigment. . . . When Fe(II) content is not less than 10 wt %, all the samples are black although a slight difference is noted in the blackness degree. When the Fe (II) content reduces to not more than 10 wt % each sample turns from black to reddish brown." The Co-coated type magnetite particles which were decreased in Fe.sup.2+ content after allowed to stand for a certain period of time are reduced in blackness degree, and when such Co-coated magnetite particles are used, it is impossible to obtain magnetic recording media with sufficiently low light transmittance.
Many attempts have been made for improving magnetic and chemical stability of Co-coated magnetite particles. For example, a method has been proposed in which a preliminary coat composed of Fe.sub.3 O.sub.4 or a non-stoichiometric compound is formed on the surfaces of core particles of iron oxide represented by the formula: FeO.sub.x (x=1.33-1.5) and an additional coat of cobalt hydroxide is formed on the said preliminary coat (Japanese Patent Application Laid-Open (Kokai) No. 63-74920 (1988)).
Thus, the magnetic iron oxide particles having high coercive force, black in color and also stable magnetically and chemically are desiderated in the art at present. The magnetic iron oxide particles disclosed in the above-mentioned Japanese Patent Application Laid-Open (Kokai) No. 63-74920 hardly satisfy all of the said properties required.
The magnetic iron oxide particles disclosed in the above-mentioned Japanese Patent Application Laid-Open (Kokai) No. 63-74920 are designed to minimize the decrease of coercive force by the temperature and quite different in action and effect from the present invention which is intended to afford a reversibility to coercive force in the magnetic iron oxide particles after heating and to improve magnetic stability of coercive force and saturation magnetization and chemical stability of Fe.sup.2+. Also, the magnetic iron oxide particles described in the abovementioned Japanese Kokai are less in Fe.sup.2+ content and low in blackness degree, and the magnetic recording media produced by using such magnetic iron oxide particles are high in light transmittance and poor in magnetic and chemical stability due to the properties of the magnetic iron oxide particles used.
The technical problem of the present invention, therefore, is to obtain the magnetic iron oxide particles having a high coercive force, and being black in color and stable magnetically and chemically.
As the result of strenuous studies of the present inventors, it has been found that by adding a mixed solution containing Co.sup.2+ and Fe.sup.2+ or a suspension containing hydroxides of Co.sup.2+ and Fe.sup.2+ in an amount of from not less than 2 moles to less than 3 moles of Fe.sup.2+ based on one mole of Co.sup.2+, to an alkaline dispersion with pH of not less than 10 in which Fe.sub.3 O.sub.4 particles are dispersed, while passing an oxygen-containing gas at a temperature between 50.degree. C. and boiling point, in such a way that the total concentration of Co ions and Fe ions in the alkaline dispersion is in the range of more than 0.005M to less than 0.5M, the obtained spinel-type composite particle comprises spinel-type Fe.sub.3 O.sub.4 particle as a core particle thereof and a spinel-type CoFe.sub.2 O.sub.4 outerlayer of the spinel-type composite particle, and is black in color, high in coercive force and stable magnetically and chemically. The present invention has been achieved on the basis of this finding.